Science / Medicine : Close-Up of Venus : Magellan Probe Due for Launch From Shuttle
Venus has always been mysterious. Nearly identical in size to the Earth, but permanently wreathed in clouds, the planet has invited intense and sometimes wild speculation about its unseen surface. Science-fiction authors, and even some early scientists, painted word pictures of impenetrable swamps, towering forests, exotic beasts and green-skinned beauties.
Reality, alas, is somewhat less colorful. With a temperature of 900 degrees Fahrenheit, the planet has no liquid water and its atmosphere, mostly carbon dioxide, is 90 times as dense as Earth’s. There are no swamps, no vegetation, and most likely not even any bacteria. Edgar Rice Burroughs’ John Carter would not find it hospitable at all.
But scientists are fascinated by Venus nonetheless. Earth and Venus “are identical except for the way they evolved,” according to R. Stephen Saunders of the Jet Propulsion Laboratory in Pasadena. “It is an ideal model for understanding planetary processes on Earth.”
In particular, the surface of Venus is covered with massive tectonic plates similar to those that support the continents and oceans of Earth. On Earth, however, “erosion blurs the tectonic processes,” Saunders said, while on Venus the processes should be much clearer and easier to understand.
Simply put, studying Venus may give scientists a clearer picture of the processes that cause earthquakes in Los Angeles.
In order to aid that understanding, the National Aeronautics and Space Administration plans to launch the Magellan probe to Venus from the shuttle Atlantis this Friday. Magellan, a patchwork of parts from other projects, will be the first deep-space probe launched by the United States in nearly 11 years and the first ever launched from the space shuttle. It is designed to map at least 80% of the planet’s surface with a resolution 100 times better than that provided by Pioneer Venus, which reached it in 1978, and 10 times better than provided by the Soviet probes Venera 15 and Venera 16.
Among the features revealed by the earlier probes are two large plateaus, each about the size of the United States, called Ishtar Terra and Aphrodite Terra. At the eastern end of Aphrodite Terra are two great rift valleys about 170 miles long and a little over a mile deep. Another prominent feature is the Beta Regio formation, which appears to be two huge volcanoes that rise nearly 2 1/2 miles above the surrounding plains.
The rest of the planet seems to be covered with an irregular rolling plain possibly spotted with some sizable crater-like formations and possibly other, smaller volcanic mountains.
Resolution from the earlier probes, however, is comparable only to that of a desktop-sized globe of the Earth. Magellan, in contrast, will be able to resolve surface features as small as 360 feet in diameter. That resolution is sufficient, for example, to determine whether small craters on the planet’s surface were caused by meteor impact or volcanoes--a key question if scientists are to understand the movements of the tectonic plates.
If all goes well, said Magellan project manager John Gerpheide of JPL, the satellite will return more data to Earth than all previous planetary probes combined.
That is a tall order for a probe that was twice rejected by the Reagan Administration as too expensive and that was finally approved only when researchers agreed to cobble it together from leftover bits and pieces of other spacecraft. The main structure and propulsion system are from Voyager, while the main computer and internal power system are spares from Galileo.
Other components come from the shuttle, the Viking Mars orbiters and landers, and the Ulysses solar mission. Only about a third of Magellan is specially built for this mission, but the total cost is still $325 million. The craft was assembled by Martin-Marietta Co. and the radar was built by Hughes Aircraft Co.
And yet this hodgepodge of seemingly ill-matched components is designed to carry out an extremely sophisticated mission once its solid rocket motors blast the 8,000-pound probe out of Earth’s orbit six hours and 18 minutes after the shuttle is launched.
Magellan will be launched into what is known as a “type 4” orbit that will take it 1 1/2 times around the sun before it reaches its target in August, 1990. A more direct four-month path to Venus will be available this October, but that launch period is allocated to the Galileo mission to Jupiter. Mission controllers opted for the longer route rather than wait for the next optimum window in 1991.
Once Magellan reaches Venus, its solid fuel rockets will slow it to a three-hour, nine-minute elliptical orbit that carries it between a maximum altitude of 4,970 miles to only 155 miles above the surface. After 18 days, during which scientists will check out Magellan’s performance, it will begin mapping.
Each time the satellite swoops down close to the surface, its synthetic aperture radar will sweep across a swath 10 to 17 miles wide. The radar sends out several thousand pulses of radio energy each second, measuring the time it takes the signal to be reflected back to the satellite and using that data and altitude information to construct a two-dimensional map of the surface.
That data will be stored temporarily in on-board tape recorders. After the radar has finished on each orbit, three gyroscope-like reaction wheels rotate the spacecraft so its antenna points back toward Earth. For 57 minutes--as it ascends back to its high point in orbit--it will radio information to one of the three Deep Space Network stations in Barstow; Madrid, Spain, and Canberra, Australia.
When Magellan reaches the highest point of its orbit, it stops playing back data and rotates to find a specific star and thereby get a fix on its orientation. It then turns back toward Earth and transmits data for another 57 minutes. The entire cycle then begins again with the next orbit.
That process will continue for one full Venus day, which is 243 Earth days long. At the end of that period, Magellan will have mapped as much as 80% of the planet’s surface. Unmapped will be the two polar regions and a broad swath, covered during orbits 685 to 830, during which the Sun will be located between Earth and Venus, preventing data transmissions from reaching us.
Once the first mapping cycle is completed, researchers hope to go back during what they call the extended mission and fill in the gaps if the spacecraft is still functioning.
The craft will also remap areas that scientists have identified as interesting in the first pass. If these areas are imaged from a slightly different vantage point than during the first pass, the researchers will be able to assemble stereoscopic views.
Magellan will carry enough reactant in its maneuvering thrusters to last for five Venus days.
Also during the extended mission, a gravimeter will be used to make highly sensitive measurements of Venus’ gravity to detect possible areas that are denser than normal, perhaps indicating subsurface ore concentrations. For these measurements, the craft’s main antenna must be pointed at Earth during the experiment, so imaging will not be possible.
Magellan Making its way to Venus--The journey will take 15 months because the spacecraft will make more than 1 1/2 revolutions around the sun before it reaches Venus.
Magellan’s Capabilities--This Magellan spacecraft is equipped with an imaging radar, the first scientific instrument that can make high-resolution maps of Venus by piercing through the thick clouds that cloak the planet.
Mapping Process 1. During the lowest portion of its polar elliptical orbit, Magellan will map the surface of Venus in 17-mile-wide swaths.
2. After the mapping phase is completed, three gyroscope-like wheels rotate the antenna back toward earth.
3. At this point, the Magellan begins transmitting data back to one of three Deep Space Network stations here on Earth.
4. About halfway through its transmitting process, at its farthest point from the planet, the craft stops transmitting data rotates and fixes itself on a nearby star to calibrate its attitude control system to make sure it’s on course.
5. Magellan makes its final transmissions to earth.
6. Finally, the craft rotates back toward Venus to begin the three-hour, nine-minute elliptical orbit again.
